How Do Earthquakes Happen? • An earthquake is defined as a sudden tremblingin the ground caused by something happening in the crust. • Most happen at the edges of plates, or plate boundaries. Very few happen in the center of plates. • About 80% happen along the edges of the Pacific plate. • Caused by plates colliding, sliding, or pulling apart. The rocks bend and stretch.
When plates move… • Great forces are exerted on the rocks. • First, the rocks bend and stretch. • Once they reach their limit, they break. • Faults form at or below the surface of the crust.
Faults • When rocks in the crust reach their breaking limit and crack. San Andres Fault
Earthquakes in NJ Why do most earthquakes occur in the mid-northern section of the state?
Normal Fault • Happens at divergent boundaries • Where plates pull apart • Rocks above the fault surface move down. • Ex: Sierra Nevada in CA
Reverse Fault • Happens at convergent boundaries • Where plates push together • Rocks above the fault move upward • Ex: Himalayas in India
Strike-slip Fault • Happens at transform boundaries • Plates slide past each other without moving up or down. • Where rocks slide past each other in different directions. • Ex: San Andreas Fault in CA
How Do Earthquakes Make Waves? • An earthquake starts the moment rocksbegin to scrape past each other along a fault. • It may be a new fault that forms at that moment or an oldfault that has already been formed.
Focus • The point where the earthquake starts, where rocks begin to slide past each other • It is usually below the surface • The sudden motion causes vibrations to spread out from the focus • These vibrations travel through the crust in the form of waves
Seismic Waves • The vibrations travel through the crust. • They reach Earth’s surface at a point directly above the focus called the epicenter.
Epicenter • Definition: The point on Earth’s surface directly above the focus of an earthquake • People can first feel the ground shaking. • This shaking is what causes most earthquake damage because it is so close to the focus. • As waves travel away from the focus they get weaker.
Aftershocks • After the first shaking, it may be quiet, and then aftershocks occur. • Aftershocks continue the damage of an earthquake. • Additional shaking and damage after the first shaking and relative quiet.
Earthquake Waves • Vibrations that are given off by an earthquake and travel through the crust are called seismic waves. • A seismic wave starts with shaking caused by rocks scraping against each other. • This shaking results in several kinds of seismic waves. • The waves travel at different speeds. • Three kinds: • Primary Waves – P waves • Secondary Waves – S waves • Surface Waves
Primary Waves – P waves • http://www.youtube.com/watch?v=2rYjlVPU9U4&feature=player_embedded • Accordion-like motion where rock squeezes together and spreads apart repeatedly. • Produces seismic waves that move in the same directionthat the rock is shaking. • The fastest seismic wave • The first to reach any faraway location
Secondary Waves – S Waves • http://www.youtube.com/watch?v=en4HptC0mQ4&feature=player_embedded • Shaking like a ruler held off the edge of a desk and “twanged” • Produces seismic waves that move in a different direction from the vibration • Travel slower than primary waves • They are the second to arrive at any given faraway location.
Surface Waves • Causes Earth’s surface to heave up and down like an ocean wave, or sway from side to side. • Tears apart structures built on the surface as the land stretches and compresses.
When a 7.0-magnitude earthquake rocked the south island of New Zealand one train track was permanently altered in what looks more like a Photoshopped image or a cartoon gag than an actual photograph.
Seismograph • Definition: A sensitive device that detects the shaking of the Earth’s crust during an earthquake. • Used by scientists to study earthquake waves • Shows patterns in the waves. • Can identify the P waves, S waves, and surfacewaves from studying the readings of the seismograph.
Seismograph • http://www.youtube.com/watch?v=83GOKn7kWXM&feature=player_embedded • http://www.youtube.com/watch?feature=player_embedded&v=DX5VXGmdnAg
Ocean Waves • When an earthquake’s focus is beneath the ocean floor, the seismic waves can travel through the ocean. • They can produce huge ocean waves called tsunamis
Tsunami Formation http://www.youtube.com/watch?v=aHljDIDf6js • When tsunamis reach a shoreline, the top of the wave travels faster than the bottom and they can reach heights of more than 50 ft. • They destroy everything in their path. • Can reach speeds up to 560 mph.
Richter and Mercalli Scales • Pg. D25 in textbook • http://www.iknowthat.com/mhscience/Earthquakes/earthquake_movie.html
Latest Earthquakes • Latest Earthquakes in the World - Past 7 days • http://earthquake.usgs.gov/earthquakes/recenteqsww/ • Earthquakes around the world • http://www.guardian.co.uk/news/datablog/interactive/2011/feb/22/earthquake-map-world